68 HOLLICK. 



turned when the ice receded, only such species becoming estab- 

 Hshed, however, as could exist under the changed conditions. 

 Fluctuations of level occurred ; the final epoch, extending to the 

 present time, being one of depression, during which the strip of 

 land was gradually disintegrated and separated into a series of 

 islands, some of which exist to-day as Long Island, Block 

 Island, Martha's Vineyard and Nantucket, while the basin of 

 Long Island Sound became filled with salt water. 



If we consider the geological features of these islands and 

 compare their floras, we may note that all except Block Island 

 still have a greater or less area of the plain region remaining 

 with them, upon which a characteristic flora finds a home. 

 Block Island has lost all of its plain region and accompanying 

 flora and is now merely an isolated portion of the terminal 

 moraine, with small areas of modern sand-beach and dune for- 

 mations, affording a home only for such species as can exist 

 under those conditions. We may thus understand one of the 

 causes which has determined the location and character of the 

 flora and one of the reasons why it is so limited in the number 

 of its species. 



Further than this, if the submarine contours of the vicinity 

 are studied it will at once be seen that a deep channel extends 

 almost entirely around Block Island. This fact is especially 

 emphasized if the twenty-fathom contour is traced out and con- 

 tinued around our coast line from Cape Ann to Staten Island, 

 (See accompanying chart, plate IX.) From such a tracing the 

 fact is evident that if we could imagine the coast to be subjected 

 to elevation, until the twenty-fathom contour became the coast 

 line. Block Island would yet remain an island, or perhaps a 

 peninsula-like projection connected with the eastern end of Long 

 Island by a narrow isthmus, while Martha's Vineyard and Nan- 

 tucket would be part of the mainland of New England. 



The indications therefore are that Block Island was the first 

 portion of the strip of land to be isolated and converted into an 

 island. The flora of the plain region, coming largely from the 

 south and possibly always having existed close to the ice front, ^ 



1 It is well known that the floras of many regions where glaciers occur, exist and 

 flourish, not only up to the ice front, but even upon the debris covering the ice. 



